It has long been known to employ phenol-formaldehyde resins as adhesives used in the manufacture of particulate wood products such as flakeboard and plywood. However, these adhesives are very expensive in proportion to the fibrous materials employed and this results in higher costs passed on to the consumer. This is particularly a problem with respect to phenol which is derived from petroleum, and thus is presently quite costly. In this regard, it has more recently been known to use certain lignins to replace some of the phenol in such resin adhesives in order to improve various characteristics in particulate wood products and to make these products less expensively.
At present, there are two main methods used in the development of lignins used to replace a portion of the phenol in resin adhesives for particulate wood panels and other products. One of these methods is the production of lignin sulfonates through the use of spent sulfite liquor from sulfite pulping. Examples of lignin sulfonates produced in this manner are disclosed in U.S. Pat. Nos. 3,956,207 (Blackmore et al.), 3,931,070 (Bond et al.), and 3,597,375 (Ludwig et al.). The other major method of producing lignins for phenolic resins comprises isolating solids from kraft black liquor following kraft pulping techniques, such as disclosed in U.S. Pat. Nos. 4,113,675 (Clarke et al.), 4,306,999 (Adams et al.), and 3,864,291 (Enkvist). The lignins produced in these two ways, however, suffer several drawbacks. For instance, sulfite and kraft lignins contain high levels of sulfate and carbohydrate residues. These residues do not contribute to useful polymer formation and serve only as fillers. Both kraft and sulfite liquors have relatively high amounts of sulfur residues which can generate noxious odors during resin preparation and flakeboard manufacture. Additionally, lignin sulfonates from sulfite liquors and sulfonated kraft liquors are water soluble, and thus contribute to the deterioration of the flakeboard in external applications due to leaching of the adhesive by water.
What is desired is to provide a phenol-formaldehyde resin which employs a lignin to replace a significant portion of the phenol, yet wherein the lignin so employed has low levels of sulfate and carbohydrate residues, has low amounts of sulfur residue, and is insoluble in water. It is further desired that such a resin will be equal or superior to other lignin-phenol-formaldehyde resins in terms of characteristics such as durability and bond strength when used as an adhesive for particulate wood products.